Various types of devices have heretofore been proposed as barrier mechanisms (optical system barrier devices) configured to open, close, and protect an objective surface of an optical system in an optical apparatus such as a camera provided with a motor-driven movable lens tube, and to protect the optical system against dust or stains caused by fingers. For example, Patent Document 1 discloses a structure characterized in that: the structure is provided with barrier blade groups each including at least three barrier blades for the purpose of suppressing an increase in dimensions and complication of the structure, and protecting an optical system and a lens tube securely and appropriately, while securing a large opening, as well as for the purpose of achieving a simple structure, making it less likely to cause operational problems, and enabling the downsizing of the lens tube; and the barrier blades are arranged in the order of a metallic barrier blade, a resin barrier blade and a metallic barrier blade.
In the optical system barrier device of this type, a mechanism for driving a barrier blade group formed of the multiple barrier blades to open and close is generally provided with a first elastic member, such as a torsion spring, for biasing the barrier blade group so as to establish a closed state. Meanwhile, the mechanism is generally provided with: a second elastic member for driving the barrier blade group so as to establish an opened state by applying a stronger elastic force than that of the first elastic member; and a clutch mechanism for discontinuing the application of the elastic force of the second elastic member to the barrier blade group when driven by an external force.
In this case, if a torsion coil spring (hereinafter a closing spring), for example, is used for the elastic force of the first elastic member and the spring is placed coaxially with the rotating center of the blade group, the direction of the application of the biasing force of the elastic member can be always made to almost coincide with a direction of the rotation of the blades, and rotational torque accordingly can be given to the blades efficiently.
In the meantime, when installing the barrier blade group, it is necessary to define a blade chamber between two parallel planes of a barrier cover (hereinafter also referred to as a cover) and a barrier base plate (hereinafter also referred to as a base plate) placed with a certain interval in between, and to manage the positions of the respective barrier blades in the direction of the optical axis in the blade chamber, for the purpose of making the barrier blade group smoothly perform opening and closing operations while restricting the posture of the barrier blade group. With this taken into consideration, there are three conceivable layouts, as follows, when installing the barrier blades and the closing spring coaxially:
(1) to install both of the blade group and the closing spring between the cover and the base plate;
(2) to install the blade group between the cover and the base plate, as well as to install the closing spring on a part of the base plate which is closer to an image surface; and
(3) to install the blade group between the cover and the base plate, as well as to install the closing spring on a part of the caver which is closer to an object.
Among them, in the case (3), the exposure of the closing spring on an external surface is not acceptable in terms of appearance quality. The provision of a member for covering the closing spring and a portion to which to fix the closing spring is not preferable, because such a member may increase the thickness of the entire barrier mechanism.
In the case (2), the blade group and the closing spring are installed separately in the direction of the optical axis by at least an amount equivalent to the thickness of the barrier base plate. The barrier base plate needs to have a certain thickness for the purpose of securing its strength, since the barrier base plate is generally formed as a resin-molded product so as to integrally form a rotating shaft of the blade group, a portion for retaining a driving member, rails for guiding slides of the blades, and so forth on the barrier base plate. For this reason, the barrier blade or the closing spring needs to have a long stretched arm in the direction of the optical axis in order for a driving force to be transmitted by bringing the closing spring and the blade group into contact with each other. The stretched arm from the barrier blade is not preferable in terms of operation stability, because such an arm increases moment for inclining the barrier blades relative to the rotating shaft upon receipt of the driving force. On the other hand, the stretched arm from the closing spring is not preferable in terms of operation stability, either, because such an arm increases moment for twisting the spring, and thus deforms the spring. Moreover, both cases are not desirable in terms of dust prevention, because: the rotation of the arm while kept in contact requires a cutout portion to be provided in the barrier base plate in a broad range for the purpose of allowing the arm to pass through: and accordingly, dust becomes easy to go into a lens tube.
In the case (1), a blade rotating shaft for retaining the closing spring and the barrier blade group is formed as a stepped boss, whose first step restricts the displacement of the closing spring in a direction at a right angle to the shaft, and whose second step, smaller in diameter than the first step, restricts the displacement of the barrier blade in the direction at a right angle to the shaft. Moreover, a position of the barrier blade group in the direction of the optical axis is restricted to the space between an end surface of the first step and an opposed surface of the barrier cover. However, the closing spring and the barrier blade group have a layout in which the closing spring and the barrier blade group are adjacently placed on a stepped rotating shaft.
In this case, a height of the blade chamber formed by the end surface of the first step and the opposed surface of the barrier cover needs to be set at a value obtained by adding a certain clearance to a total thickness of the blade group so that the barrier blades can slide smoothly. Accordingly, when a load is applied to the closing spring so as to be pressed against the shaft, the closing spring is sandwiched between the barrier blade and the end surface of the first step and the barrier blades are gathered together to the cover due to backlash. This hinders the stable operation, and makes it difficult to protect the photographic lens.